This invention relates to the manufacture of corrugated paperboard sheets and, more particularly, to the control of warp developed during the manufacture of such sheets.
The standard procedure for the manufacture of corrugated paperboard sheets for use in the packaging industry, involves first forming a continuous composite paperboard web composed of a corrugated paper medium interposed between a top paperliner and a bottom paperliner and provided with a moisture-laden heat-setting adhesive material at the juxtaposed surfaces of the component layers of the web. The thus-formed paperboard web is then passed through a heater where it is heated to a temperature sufficiently high to effect vaporization of moisture from the adhesive material to thereby set the adhesive material and firmly bond together the juxtaposed surfaces of the component layers of the web. The resulting bonded paperboard web is thereafter passed through a cutting mechanism where it is cut into individual paperboard sheets, which are then collected into stacks.
One of the most frustrating problems which has continually plagued the paperboard manufacturer is that of warp in the finished paperboard sheets, which oftentimes reaches a level of severity requiring the scrapping of a substantial portion of the sheet production. Although the causes of warp in corrugated paperboard production have been extensively studied and documented, and various techniques have been proposed for minimizing such causes, a truly effective means for controlling warp to the point of ensuring the production of consistently substantially flat corrugated paperboard sheets still remains the seemingly "impossible dream" of the paperboard manufacturing industry.
It is generally accepted that warp is caused by a number of contributing factors which are developed within the paperboard web prior to and during its bonding operation, and which lead to nonuniform changes in the dimensions of the components of the paperboard structure, particularly the top and bottom liners, which take place subsequent to the bonding operation when there can be no further relative movement between such components. All of these various factors contributing to warp are inherent in the raw materials and procedures employed in forming the bonded paperboard web. Such contributing factors include differences between top liner tension and bottom liner tension, differences in moisture content between the top liner and bottom liner rollstocks, the relatively large amounts of moisture introduced into the web with the adhesive material (which generally contains about 80% water), moisture variations introduced into the web due to nonuniform adhesive application and adhesive rheology, moisture content differentials between the component layers of the web brought about by the bonding operation (which generally involves the direct application of heat to only the bottom liner side of the web, thereby driving mositure from the bottom liner and adhesive up into the corrugated medium and top liner), and the fundamental characteristic of paper on being wetted and then dried to shrink to less than its original length. The presence of these various contributing factors in any of a wide variety of different combinations and degrees leads to the development within the paperboard web of a totally unpredictable set of conditions, the net effect of which could result in producing in the finished paperboard sheet, leaving the corrugator machine, any one of the various types of warp designated in the industry as MD (Machine Directional) up-warp, MD down-warp, CD (Cross-Machine Directional) normal-warp, CD reverse-warp, twist-warp (a combination of MD and CD-warp), and S-warp (a combination of up or normal warp and down or reverse warp).
The previous proposals for controlling warp in corrugated paperboard manufacture, for the most part, have been directed toward attempting to eliminate one or more of the various individual factors contributing to the warp-producing conditions developed within the paperboard web prior to and during its bonding operation. For example, it has been suggested to replace the heat-setting adhesive with a cold-setting adhesive which would not require the application of heat to effect the bonding operation, but this approach has not proved successful due to the lack of any suitable cold-setting adhesive which would in any way compare in performance and economy with the conventionally employed heat-setting adhesives. Other proposals have included trying to eliminate tension differences between the two liners by synchronizing liner speeds; trying to equalize moisture contents of the two liners by treating the individual liners with pre-heaters or water sprays; and trying to improve control of adhesive application. While each one of these proposed systems may reduce or even substantially eliminate a single one of the several factors contributing to warp, it still has not been possible, even when employing all of these proposed systems in combination, to eliminate all of the warp-causing factors and ensure the production of consistently substantially flat paperboard sheets. Moreover, employing a separate control system for dealing with each individual warp-causing factor results in a rather costly installation and hence, would not appear to be the best approach for solving the warp problem from an economical standpoint.
A somewhat different approach to the warp problem in corrugated paperboard is described in the Taylor, U.S. Pat. No. 3,826,178, which, instead of being concerned with an on-line technique for controlling warp before it has actually occurred, relates to a procedure for treating individual corrugated paperboard sheets, off of the corrugator to correct the warp after it has occurred. At this stage of the manufacturing operation, the board is essentially "cold" board, i.e., it has already cooled down from its heat-bonding temperature to room temperature or slightly above, typically within the range of from 80 to 100.degree. F. The Taylor procedure employs a water application roll which applies a controlled amount of water to the convex side of the individual warped sheet of corrugated paperboard off of the corrugator. According to the patentee, when the fibers on the wetted convex side of the sheet dry, they shrink to a greater extent than they had originally, thereby pulling the sheet to a non-warped flat condition. There are several drawbacks to the Taylor procedure which seriously detract from the practicality of employing it as a warp control means in high-speed commerical corrugated paperboard production. First of all, this procedure requires application to the paperboard sheet of relatively large quantities of water, i.e., on the order of approximately 25 to 30 wt% based on the weight of the liner component on the treated side of the paperboard sheet. Since cold board treated with this quantity of water will not reach equilibrium moisture-content conditions with the ambient atmosphere and thereby obtain dimensional stability for a period of from 1 to 2 days, the operator must wait for this period of time in order to determine if he has applied the precise amount of water to correct the warp. Such time lag greatly increases the likelihood of either an undercorrection or an overcorrection of the warp, this latter situation resulting in the sheet becoming warped in the opposite direction from that of the initially treated condition (e.g., an initial CD normal-warp will be converted into a MD down-warp). Additionally, due to the fact that cold board does not readily absorb moisture, stacking of the water-treated cold board will result in moisture being transferred thereform to the dry side of the adjacent board in the stack, thereby influencing the dimensional changes taking place in the adjacent board and further complicating the predictability of the effectiveness of the treatment. Furthermore, the large amounts of water required by this treatment and the long time lag required for the board to dry to a dimensionally stable condition result in other problems, such as dimpling of the board surface making it difficult to register on a printing roll, reduction of the flat crush strength of the board, and difficulty in getting ink to adhere to the still wet board surface.